A first type of undercarriage is known, called “rocker-beam undercarriage” furnished with an undercarriage leg attached to the structure of the rotorcraft via an attachment, in order to be able to make a rotary movement about this attachment.
This landing gear leg comprises a damper and a running assembly provided with at least one wheel.
Furthermore, the undercarriage has a retraction cylinder sometimes called “stay rod cylinder” by those skilled in the art.
The purpose of the retraction cylinder is therefore to retract the undercarriage in order to take it into a compartment of the rotorcraft when the latter is flying or in order to take it out of this compartment during landing, causing the landing gear leg to pivot about its attachment.
In addition, the retraction cylinder is sometimes stressed horizontally, namely in a direction substantially parallel to the ground, in order to keep the landing gear leg out when the rotorcraft runs on the ground.
More precisely, the retraction cylinder keeps the landing gear leg out irrespective of the obstacles, a stone, for example, encountered by the rolling assembly of the undercarriage.
Conversely, the damper is stressed only vertically because the function of this damper is to damp the impacts applied to the undercarriage during a landing.
Consequently, the undercarriage is conventionally fitted with a damper stressed only by vertical impacts. In addition, the undercarriage comprises a retraction cylinder which makes it possible on the one hand to take the undercarriage in or out of a compartment and, on the other hand, to keep the landing gear leg of the undercarriage in position when this landing gear leg is out of the compartment, independently of the horizontal stresses exerted on the landing gear leg.
In addition, a retraction cylinder is usually furnished with a retraction chamber and an extension chamber and a piston sliding in a cylindrical sheath, the retraction and extension chambers being separated by the head of the piston.
Furthermore, the retraction and extension chambers are connected to a hydraulic circuit.
To retract the landing gear leg, a control member of the rotorcraft injects a fluid, oil for example, into the retraction chamber through a retraction port. The pressure exerted by this fluid on the piston head causes this piston to retract.
Furthermore, since the piston is articulated on the landing gear leg, a retraction of the piston causes a rotary movement of this landing gear leg and therefore causes it to retract into the rotorcraft compartment provided for this purpose.
Conversely, in order to take out the landing gear leg, a control member of the rotorcraft injects a fluid into the extension chamber, through an extension port, so that the piston can come out of the body of the cylinder in order to push the landing gear leg out of the compartment.
Furthermore, when the landing gear leg is in the desired position, the retraction and extension ports are locked, via a solenoid valve for example.
The pressures exerted by the fluid in the retraction and extension chambers are then maintained at a constant value, because of the hydraulic locking achieved with the aid of the solenoid valves, which immobilises the position of the piston.
When the landing gear leg is out, this landing gear leg will sustain a horizontal stress when encountering an obstacle, a stone for example, which is necessarily transferred to the retraction cylinder.
Since the piston is locked, this piston cannot move and then damp the horizontal stress sustained by the undercarriage.
Note that this first type of undercarriage is very common in the aviation field since it satisfies the needs during flights operating in normal conditions.
Nevertheless, in the event of a failure of the rotorcraft, it is possible that the vertical speed of this rotorcraft, for example in autorotation, namely very high, this vertical speed then greatly exceeding normal landing speeds.
It is noted that the damper of the undercarriage no longer makes it possible to damp the impacts due to the impact of the undercarriage on the ground, the power of vertical shock absorption of the undercarriage then being exceeded.
It is easy to understand that this situation may lead to unfortunately catastrophic events, the undercarriage no longer being able to respond to the needs notably in the case of a crash of the rotorcraft.
To remedy this, it is conceivable to design the damper in order to allow it to withstand extreme vertical stresses. However, the space requirement of the damper, its weight and its cost then become prohibitive.
Document FR2608242 describes a second type of rocker-beam undercarriage furnished with a landing gear leg attached to the structure of the rotorcraft.
In addition, the undercarriage is henceforth provided not with a damper and a retraction cylinder but with a damper-cylinder.
The damper-cylinder is substantially perpendicular to the ground, while the landing gear leg is substantially parallel to the ground when this landing gear leg is retracted into a compartment of the rotorcraft.
The damper-cylinder then fulfils the functions of a conventional damper but also those of a retraction cylinder, the damper-cylinder finally comprising a retraction cylinder sliding in a damper.
This second type of undercarriage is practical because of its simplicity but may have the same disadvantages as the undercarriage of the first type.
In addition, it is noted that it is not possible to arrange a damper-cylinder on the first type of undercarriage.
Specifically, if the damper of the first type of undercarriage is replaced by the damper-cylinder, it will be appropriate to retain the retraction cylinder in order to retract the landing gear. Since the damper-cylinder is incorporated into the landing gear leg, this damper-cylinder cannot retract this leg.
Furthermore, if the retraction cylinder of the first type of undercarriage is replaced by the damper-cylinder of the second type of undercarriage, the situation will remain unchanged. The damper-cylinder will be able to retract the landing gear leg but will have no impact-damping function during a high-speed landing because this damper-cylinder is stressed only horizontally.
However, document FR 2687123 provides a solution to solve the latter disadvantage.
The landing gear leg of the undercarriage is then fitted with a wheel whose spindle, that is to say the rotation shaft of the wheel, is offset relative to the longitudinal axis of the landing gear leg.
In addition, the landing gear leg is furnished with a damper-cylinder comprising both a retraction cylinder and a damper.
The vertical stresses sustained by the landing gear leg then generate a torque which tends to retract this landing gear leg.
However, the damper-cylinder is then designed so that the said torque is less than the stresses that can be absorbed by this damper-cylinder.
Consequently, to be able to be effective in the case of a crash of the rotorcraft, the damper-cylinder would have to be designed to withstand extreme horizontal stresses which would make its space requirement, its weight and its cost prohibitive.
In addition, it is relatively difficult to perfect a damper-cylinder comprising in series a retraction cylinder and a damper.